S H 



RADICAL PREVENTION OF COSTIA 
NECATRIX IN SALMONOID FRY ^ ^ 



From BUI.I.ETIN OF THE BUREAU OF FISHERIES, Volume XXVIII, 1908 



Proceedings of the Fotirth International Fishery Congress : : Washington, 190S 





WASHINGTON 



GOVERNMENT PRINTING OFFICE :::::: 1910 




Class S>-iH 115 

Book • 6l>F^ 



RADICAL PREVENTION OF COSTIA 
NECATRIX IN SALMONOID FRY ^ ^ 

From BULLETIN OF THE BUREAU OF FISHERIES, Volume XXVIII, 1908 
Proceedings of the Fourth Internatioiial Fishery Congress : : Washington, igo8 




[^ 



WASHINGTON ::::;: GOVERNMENT PRINTING OFFICE 



1910 



r V r ^ 



BUREAU OF FISHERIES DOCUMENT NO. 690 
Issued April, 1910 



APR lb 1910 



RADICAL PREVENTION OF COSTIA NECATRIX IN 
SALMONOID FRY 



By Johann Franke 

Director oj the Fish-Culture Establishment at Studenec and Secretary of 
the Fishery Committee for the District of Krain 

Paper presented before the Fourth International Fishery Congress 
held at Washington, U. S. A., September 22 to 26, 1908 



917 



CONTENTS. 

Page. 

Characteristics cif the disease 919 

Accidental development of the preventive method 920 

Experience of the season of 1906 922 

Precautions applied in 1 907 924 

The season of 1 908 925 

Experiments in the hatching house 925 

Experiments in the pond _- - 927 

Summary and conclusion _ _ 928 

918 



RADICAL PFIEVENTION OF COSTIA NECATRIX IN 
SALMONOID FRY. 



By JOHANN FRANKE, 
Director of the Fish-Culture Establishment at Studenec and Secretary of the Fishery committee 

for the District of Krain. 

[Translated from the German] 
CHARACTERISTICS OF THE DISEASE. 

I insist upon the limitation to "salmonoid fry," because I have not directly 
observed Costia, nor have I seen the characteristic exterior appearances of 
costiasis, on any adult fish with one exception. I saw four years ago in June, 
in the Stara Voda, in a broad place in the stream where the current was very 
slow, a pike some 23 centimeters in length with a whitish covering on the 
skin resembling a veil, very like figures 12 and 13 in Dr. Bruno Hofer's 
"Fischkrankheiten," in which work appears a full description of this disease." 

The place where my observations were made was the fish-culture establish- 
ment at Laibach, Austria. 

The appearance of Costia was noticed among the fry some five to ten 
days after they had begun to feed, i. e., after the resorption of the sac — never 
before this period — and equally whether the fry began to feed early or late, 
among the early feeding Salvelinus jontinalis and alike the late Salmo irideus. 
About the middle of June, sometimes ten days earlier, all trace of Costia dis- 
appeared as mysteriously as it had come. I have no reliable criterion as to 
whether the fish became immune against costiasis in June or whether Costia 
in the form of a flagellate is seasonal, but I suppose the latter to be the case, 
since the signs of disease disappear at the same time among the younger and 
older fry. 

No difi'erence could be found in the susceptibility of the young fishes; the 
fry of the three species regularly cultivated — Salvelinus fontinalis, Salmo fario, 
and Salmo irideus, obtained from brood fishes of the establishment (among 
which may be included the 100 kilograms of Salvelinus fontinalis and Salmo 



a Hofer, B.: Handbuch der Fischkrankheiten, p. 115-121. Munich, 1904. 

919 



920 BUIvLETIN OF THE BUREAU OF FISHERIES. 

fario from the excellent stream of Stara Voda) — were attacked along with the 
embryonated eggs obtained from elsewhere." The fry that were fed nothing 
but live crustaceans and larvae of mosquitoes, their natural food, were infected 
as much as those which, on account of temporary lack of natural food, were fed 
partly with substitutes, such as pig liver or beef and veal spleen. 

The infection must, consequently, be of the locality. The place, the water 
and its near surroundings, must be infected, the shores harboring Costia in the 
shape of cysts on the dry land, whence they are scattered everywhere by 
the wind. 

Costia had already established itself at Studenec before my arrival in 1891. 
Costiasis thus did not begin during my direction, but it spread so rapidly and at 
last in such manner that none of the springs were safe from it. It was first rec- 
ognized in 1904 by Dr. Ivan Robida, head of the hospital for the insane in 
Studenec, who was fond of the sport of fishing and who in his close relations 
with myself studied questions which interested me. By means of his micro- 
scope (my own not powerful enough) and Dr. Hofer's book, the identity of the 
disease germ was fixed in 1905. 

We conclude, further, that we have found the cause of the abnormal mor- 
tality of fry in previous years, there appearing the same phenomena and 
symptoms and course of the disease from the very beginning that had char- 
acterized those great losses for which no cause was known from 1S96 until 
this time. I had sufficient occasion and opportunity to observe all the 
phenomena and symptoms minutely, and likewise to remember them, for a 
large part of the feeding fry were placed for one to three months in larger 
hatching boxes, then in floating troughs, and in September and October in 
large ponds in which to pass the winter, while I spent each year 180 half 
days and 40 to 60 entire days in this establishment. 

ACCIDENTAL DEVELOPMENT OF THE PREVENTIVE METHOD. 

The radical means of preventing Costia was not " discovered, " nor even 
"found," for it was not sought. It developed in the following manner: 

In the one-story house occupied by Doctor Robida and other physicians 
of the insane asylum there is a tank under the roof with capacity of about 
1,800 liters, into which was pumped water for household purposes from a spring 
situated in the cellar of a house about 80 meters distant, if sufficient water was 
coming to the ponds for the working of the pump. The spring in the cellar 
and the tank are well covered and the pumped water, coming in contact with 
the fresh air from without only by chance rifts in the cover, can not be much 

a-Salmo fario from Ilidze, Bosnia, 1902; Salmo dentex (Isonzo trout) from Idria, 1903-1907, 
inclusive. 



PREVENTION OF COSTIA NECATRIX IN SALMONOID FRY. 92 1 

contaminated by dust, etc. Doctor Robida took advantage of the vicinity of 
the tank to supply with water therefrom two small aquaria in his room (1905). 
From the main pipe, made of lead, the water passed into the aquaria through 
slender rubber tubes (3 millimeters in diameter at the outlet) with brass end 
pieces (about i millimeter at the opening), under pressure of about half an 
atmosphere. One of the aquaria consisted entirely of glass and had a bottom 
area of 35 by 25 and a height of 22 centimeters, while the other was somewhat 
larger and had a lead bottom and frame, with walls of glass. In the bottom of 
each was a layer of 5 to 6 centimeters of line, white, well washed, calcareous 
sand, and a few shoots of water cress were planted in it. The water, falling in 
a slender jet, boiled up actively and sent out small bubbles in every direction, 
so that even in the corners they could be seen dancing in the water. In the 
first aquarium were placed more than 300 young Salvelinus fonlinalis old enough 
to feed, while in the larger one were placed Salmo fario and irideus, also a few 
lontinalis, in all some 500 fry. More than any other fry these were fed with 
crustaceans exclusively, which were greedily devoured, especially by the Amer- 
ican species, which fed until the body swelled quite out of shape and looked as 
if it would burst. The excrements were removed daily by means of a small 
suction tube, while once each week the aquarium was thoroughly cleaned, the 
sand washed, etc., the fishes being placed in other quarters during this pro- 
ceeding. Costia had in the meanwhile appeared in the hatchery as in the 
preceding years, but there was no trace of the disease in the aquaria. 

Ten diseased Salvelinus fontinalis were now put into the smaller aquarium. 
The infection had not as yet shown its full effect on them and Costia had estab- 
lished itself microscopically on other fishes looking like these. The diseased 
fishes differed from the fat, healthy ones, not only by the thinness of body Ijut 
also by the coloring, which was more or less of a dark blackish blue hue, with a 
faint, almost invisible shading as compared to the light-colored and white 
markings of the healthy individuals, and the difference was apparent to the 
casual observer. The diseased fishes continued to live, seeking the bottom in 
the quietest places and rarely moving about, and looked at the last like a thin 
blackish thread with a thick knot. All died within 6 to 9 days after the fishes 
of the same lot and of the same appearance left in the hatchery. Expecta- 
tion as to the results of this experiment was naturally great, but no effect was 
produced on the fishes in the aquarium. 

A second experiment in the second aquarium gave the same result, and 
several more were made in each aquarium. Doctor Robida attempted to con- 
vey the infection by other means, i. e., by the infiltration of infected water and 
by the direct introduction of living Costia, but with no result. He changed the 
food freely, giving the fishes, when they had grown larger, grated meat from 



922 BULLETIN OF THE BUREAU OF FISHERIES. 

his own table, even chopped hver, thinking that in water so well aerated even 
such food could do them no harm." 

The abundant aeration of the water proved to be a radical prevention 
against Costia, all the fish remaining alive and healthy, not one being lost. 
When the action of the pump grew defective on account of scarcity of water, 
Doctor Robida used a small motor operated by alcohol for the purpose of 
obtaining a current in the aquaria. But the disadvantage increased and I put 
the fishes, which were from 3 to 4 centimeters long, into a rearing trough (8 
meters long, 0.55 meter wide, and 20 centimeters deep), merely giving them 
three more salt baths, since this was the end of the critical period, in order to 
be safe from the danger of Costia. 

I have never seen the white veil-like covering spreading over the skin, as 
shown in figures 12 and 13 in Doctor Hofer's book, except on the pike already 
mentioned ; never on the small fry. So long as I fought Costia with potassium 
permanganate and not with cooking salt, as did Doctor Hofer, the fishes which 
had withstood Costia had white fungus spots near the gill openings, and these 
spots, in spite of the treatment with potassium permanganate, were in some 
cases fatal. Since I have begun to use common salt, I have not noticed this 
last phenomenon. I suppose that the fishes attacked by Costia are too small 
and consequently too weak to endure this condition until the white spots 
show on the skin, and die before this stage. 

EXPERIENCE OF THE SEASON OF 1906. 

It was impossible to arrange an aeration of the hatchery troughs by means 
of water under pressure, on account of lack of fall in the supply. The only fall 
periodically in operation was occupied by the already mentioned pump and not 
available for hatchery purposes by reason of its location. Thus I could not put 
into practice the new experience with aeration. 

Since Costia was again to be expected in the hatchery, however, I arranged 
in a pond, which had not been used for fishes for four years and the water flow 
of which was used only to supply two rearing troughs, a place in the open for 
the hatching boxes. This small pond was repeatedly dry when the water was 
low in the springs. The bottom was cleaned of all vegetation, raked and washed 
out, highly saturated with potassium permanganate, and after this washed 
out with salt. All the small fry able to feed and destined for rearing in the 
establishment were brought to this pond. The water, as may be easily under- 
stood, never grows muddy, has a constant temperature of 9.8° C, and produces 
many green algae, which are very cumbersome when the currents of the water 
become slow with low water in the springs. 

<» In my opinion such food can not be given long, never exclusively, and of the latter sort not 

even to large fish. 



PREVENTION OF COSTIA NECATRIX IN SAI^MONOID FRY. 923 

These measures of precaution and a careful maintenance of cleanliness in 
the hatching boxes, etc., as well as the sole use of live natural food, brought 
about only the result that in the two boxes first installed the fry did not develop 
the Costia until four or five days later than in the hatching house, and that the 
infection did not spring up immediately in a violent form, but crept in upon 
them slowly and insidiously. It may be concluded thence that after the clean- 
ing and thorough disinfection of the pond, etc., the water was free from Costia, 
but was reinfected by the nonsaturated ground of the banks, from cysts which 
must have been carried into the water by the wind. But whence come these 
cysts? The following explanation readily presents itself: 

The dirt from the rearing troughs (during the first years of my direction 
there were eight of them at three different places, for the most part occupied 
by two separate lots of fish), the excrements, debris of food, and ooze from the 
algae and the grounds were washed down into the pond water; there formed 
in the wintering ponds during eight to nine months at the places where the 
water did not course so freely a thick layer of fat, black, ill-smelling ground 
ooze, and the ponds could not be cleaned except by flushing them out, scrap- 
ing, sweeping, and washing out the ground ; all this carried off into the principal 
pond. The latter can be emptied only down to about five-sixths of its contents, 
and all the springs of the local systems flow into it, through it, and off by means 
of one lock. From the principal spring, which is easily accessible to the village 
of Studenec (three or four butchers, the cattle, etc.), much organic matter comes 
into the pond; it continually receives manure from this source, and incidentally 
from the well-frequented road during rainfall. Thus a rich bottom fauna and 
very abundant vegetation develop. The latter must be taken out partially 
several times a year and thoroughly once annually. Much ooze is naturally 
taken out with the Chara fragilis, and everything taken out of the pond is 
piled on the banks in heaps, where it remains sometimes for two entire years. 
As long as the springs were full and there was a corresponding flow of water, 
a total of 600 to 800 second-liters in the maximum and never less than 200 
second-liters up to 1896, no bad effect was noticed on the fishes from the pollu- 
tion of the ground and its oxidation. And, frankly speaking, I knew nothing, 
as so many others, about the importance in fish rearing of ground culture and 
ground sanitation. When the scarcity of water and lack of currents began to 
be felt and had grown quite noticeable in 1904, and the well-known effects of 
such conditions, among others the presence of Costia, appeared in the fish-cul- 
tural work, I was forced to look for explanation and remedies. 

Conditions for the existence of Costia were rendered more and more natur- 
ally favorable by the decrease of water supply in the summer of 1905, the winter 
following, and later down to a very few liters, and by the fish-cultural opera- 
tions; and the persistence of the infection was insured by the maintenance of 



924 BULLETIN OF THE BUREAU OF FISHERIES. 

old and the establishment of new piles on the bank whence Costia cysts would 
be derived. I can not find any other explanation for the infection of the pond. 
Cooking salt was again our resort; by this means I carried through the 
critical period one-fourth of the fashes in the worst cases and three-fourths of 
them in the less severe. 

PRECAUTIONS APPLIED IN 1907. 

The same pond was again thoroughly cleaned and disinfected, then a part 
was partitioned off in a shallower portion by means of a wall of clay. The 
water for the fry flowed through a tube of rubber and lead through the dams 
into the distributing trough and thence through lead siphons into the hatching 
boxes. The covers for these were fitted better and more closely than in 1906, 
and supplied with glass openings in order to give the fishes both light and sun 
without having to take off the covers; the distributing trough was likewise 
kept covered as much as possible and the cover was lifted only for the cleaning 
of the boxes. The flow of water was increased by five, six, and eight times 
the ordinary amount for the cleaning of tlie boxes, by which means the sediment 
was whirled up, flooded through the closing screen, or deposited on the latter 
to be swept off by means of a soft brush; the whirling up and flowing off of 
the sediment was aided also by means of a feather. 

At the end of March, some ten days later, no trace of Costia was found in 
two boxes of Sahelinus foniinalis; but only fourteen days later, on April 4, 
I saw two fishes the color of which was not quite satisfactory. On April 5, two 
fishes were dead and four or five had changed color. The naked eye and the 
microscope both testified to the unwelcome truth — it was Costia again. March 
was very dry and very windy during the latter days. I gave the fishes a salt 
bath of some fifteen minutes duration on April 5 and 8. Then there was no 
trace of the disease until the i8th, when I gave another salt bath. It again 
appeared necessary to give the bath on the 20th, 24th, and 28th of April, on 
the ist, 3d, 19th, 24tli, and 28th of May, for twenty-five minutes, and, lastly, on 
June 3 for thirty minutes, when the fishes were transferred to the rearing trough. 

The covering of the water was not entirely useless, the infection in the two 
first boxes having had two long intervals, the first ten and the second twelve 
days. The three lots nearest to the outflow needed the salt bath most frequently, 
i. e., every other day without intermittence ; these were Salmo irideus of May 
3 to June 13. The explanation of this fact is the following: The cover of the 
trough had to be taken off every morning and every evening during cleaning 
time, and this admitted the dust and the cysts, caught up by the wind, which 
were brought by the current to the outflow in greater quantities than at the 
place where the water flowed in. 

I carried through the critical stage about 4,000 Salvelinus fontinalis, which 
were kept in three boxes, the last 1,400 being taken by myself on August 17 



PREVENTION OF COSTIA NECATRIX IN SALMONOID FRY. Q25 

to the Wocheiner Lake, a journey from 5.30 a. m. until i p. m., without incur- 
ring any loss; the fish were from 5 to 7 centimeters in length. 

I could not detect that the salt water did any harm to the fishes. The 
water was of course aerated incessantly by taking it up in a 2-liter vessel and 
pouring it back from a height of 50 to 70 centimeters. Only the Salmo fario 
remained at the bottom during this proceeding; the irideus and the joniinalis 
had to be kept out of the way with a gauze hand net. 

Doctor Robida did not take any part in my experiments in 1906 and 1907, 
and left Studenec last year. 

THE SEASON OF 1908. 

The drying up of the springs, which was no longer doubtful, in addition 
to the spreading of the Costia, decided those in authority to abandon the locality 
near Studenec. But my desire and hope that my fish-cultural difficulties would 
end with the year 1907 were not fulfilled, as the spawning season of the sal- 
monoids came round before measures for abandoning the locality could be taken. 

Since I could command my time, I wished to make use of my knowledge 
of the effect which the introduction of atmospheric air had upon Costia. I 
sought, first, suitable cylinders, similar to those of the Hydrobion; air was 
pumped into these and was to rise gradually from the bottom of the fish troughs 
in small bubbles to the surface. Two attempts to obtain the clay cylinders 
met with failure. 

Salt baths are good, and capable of saving the fry from entire or enormous 
losses; but they can not be lastingly effective if the water is continually 
infected anew, and they must, consequently, be repeated; and even while apply- 
ing them every forty-eight hours I had to register losses which amounted in 
time in the most favorable cases to one-fourth of the fishes placed in the basin. 
They also take much time, for a man can accomplish at the same time the 
necessary aeration of the water in but two or three hatching boxes at the most; 
ten boxes would thus demand four to five hours. 

EXPERIMENTS IN THE HATCHING HOUSE. 

The distributing trough in the hatching house stands some 48 centimeters 
above the ground and is 22 centimeters deep. I placed the hatching box on the 
floor and obtained thus a fall of 33 centimeters. A siphon having 8 millimeters 
interior diameter gives, by exact measurements, 4.2 liters per minute; the capacity 
of one box is 2,514 liters, and the water is changed therein in 5.98 (6) minutes 
with one siphon and in three minutes with two. I placed above the hatching 
box a basin 75 centimeters interior depth, containing 250 liters of water. The 
water flowed therefrom into the hatching box through a flexible rubber tube i 
centimeter interior diameter and with a conical nozzle of zinc with an opening of 



926 BULLETIN OP THE BUREALT OF FISHERIES. 

2.5 to 3.5 centimeters. The pressure of the water in the basin varied between 
130 centimeters at the maximum and 55 centimeters at the minimum. Accord- 
ing to the opening of the zinc nozzle the upper basin was emptied in fifty to 
thirty-five minutes. The falHng jet of water was so placed that the water in the 
hatching box began to rotate. The upper basin was filled two or three times 
daily for one hatching box. 

I placed Trulta lacuslris in the hatching box, fry obtained from very beautiful, 
large, eyed eggs, of which I received 5,000 from Schliersee in Upper Austria. The 
eggs were placed, on January 31, in two California hatching boxes (without the 
inner set of brass wire trays) between flat roof tiles. The first hatched fishes 
appeared on March 23 in the receiving boxes placed below. The two boxes in 
which the eggs had been placed were opened and 81 eggs were found thickly 
covered with ooze and fungus. Since the fry were very unevenly hatched, they 
were placed immediately in two clean and thoroughly darkened boxes, being 
transferred first to one and then to a second under the falling water and fed 
with live food. On April 28, when the fry hatched latest had exchanged their 
light coloring" for a darker, and fed as greedily as the older fishes, they were sent 
to the Wocheiner Lake, which they reached "in faultless condition," according 
to the report of the recipient." 

The temperature of the water used in February was 4° to 5°, in March 5° 
to 8°, but rose later to 10°, then to 13° C. Until April 12 there were no losses; 
after this there were three in all, one fish being choked by a crumb. No Costia 
was apparent. On March 30, some seven days later, the second siphon was 
set flowing for the first lot, consequently 8.4 liters of water were received per 
minute; the same was done for the second lot. 

As a control lot, on March 28, 30 fish had been placed in a small box (con- 
taining 1,362 liters of water, flow of 4.2 liters per minute) arranged as heretofore, 
i.e., on a level with the distributing trough without waterfall or increased pres- 
sure. On the 3d of April I noticed two weak fishes, one of which was found 
dead on the 4th, and I found Costia by a microscopic investigation of another fish 
showing signs of disease. After giving a salt bath to the remaining fishes I left 
them to their fate in a spring of the pond. 

I saw Costia renewed between afternoon and the next morning in a control 
lot of fontinalis during the last third of April in spite of salt baths. For security 
and my own satisfaction I gave a salt bath of 1.5 per cent of 35 minutes' duration 
to a lot of Trutta lacustris in the morning of April 25 and 27 before shipping 
them away. 

o The transportation in two casks of 128 to 132 liters lasted i>i to 2 hours by wagon, 2 hours and 
22 minutes by rail plus 47 minutes and 13 minutes standing, a total of 6 hours and 35 minutes. The 
water was cooled in the hatchery and when placed in the railway carriage was of from 10° to 7.5° C. 
The day was warm and sunny. The dimensions of all the boxes were 52 by 33 by 22 centimeters. The 
depth of the water was 15 centimeters. 



PREVENTION OF COSTIA NECATRIX IN SALMONOID FRY. 927 

EXPERIMENTS IN THE POND. 

It was not possible to arrange a waterfall there. I placed two barrels con- 
taining 200 liters, so that the water flowed, as in the hatchery house, through 
longer or shorter rubber tubes, according to necessity, and in slender jets into 
all the hatchery boxes. The board covering of the cut-off' part of the pond had 
been removed in the preceding autumn and had not been renewed. The maxi- 
mum of the pressure was 118 centimeters in both barrels, the minimum 33 
centimeters. The filling of each of the barrels took place at least twice daily, 
later even as often as five times. During the first week the water had to be led 
up from the pond over a small scaffolding, as the spring was still too weak, but 
after some rainfall the water could be pumped straight from the pond. 

On March 27 two boxes with 5. fontinalis were set up. On April 4 Costia 
was noticed among them in spite of the jet of water from the barrels. The 
daily aeration of the water for 1 14 hours to 2 hours was of too short duration 
and too little effective with the pressure obtaining. The outflow pipe of the 
barrel and the small opening of the nozzle were frequently clogged by things 
carried in by the wind and taken up by the pump. 

There were seven boxes in all and in each of these the fry received a salt bath 
of 2 per cent for thirty minutes every other day. To all appearances the aeration 
and the streaming of the water from the barrels did not remain without effect. 
The boxes could be thoroughly cleaned during the whirling of the water, and it 
could not be denied that the fishes grew more lively in the currents, darting 
through the whirls after the food without paying any attention to the fact that 
the jet of water pressed them downward; and, the most important of all, losses 
were not so frequent as heretofore and amounted (by estimate) to not over one- 
quarter in the maximum, and in a lot of 5. irideus it was very small, in fact 
inconsiderable. 

This lot came from large, beautifully colored parents. I had, however, done 
a foolish thing with the eggs. Since it is very difficult and takes a great deal of 
time to place the eggs regularly on the tiles so that they will not touch each 
other, I had ordered flat, round depressions made in regular rows in two zinc 
sheets in order to facilitate the work. The placing of the eggs was effected 
beautifully, but think of my horror to see, after opening the breeding boxes, 
instead of the hoped-for i ,900 or 2,000 fry, only 378, although these were almost 
all large and fine. Ooze had settled in the depressions with the eggs and filled 
the spaces between them. 

On April 1 4 these fry were put in the pond and were cared for more than the 
others in regard to food and aeration. Up to May i the losses amounted to 25 
fishes; up to June 3 there were only three more. After June 11 there remained 
only three boxes to be taken care of, and the above mentioned irideus were 



928 BULLETIN OF THE BUREAU OK FISHERIES. 

treated to more frequent aeration, from eight to ten times daily. Beginning 
with June 24 I ventured to omit the salt baths, and since no losses resulted I 
decided to omit the baths entirely and confine myself solely to aeration; rightly, 
too, as I saw afterwards. On June 16 the fish were all sent away in a cask, fresh 
and healthy. The cask contained 63 liters of water without ice. Duration of 
transportation, i hour and 30 minutes by wagon, i hour and 30 minutes by 
rail, 2 hours and 30 minutes by wagon, in all 5K hours. At the pond 3 fish 
were found dead, wounded by lumps of ice which were put into the water in the 
railway car without any ice bag. 

SUMMARY AND CONCLUSION. 

I come to the following conclusion from the above-mentioned experiments : 
A means for the radical prevention of Costia necairix in salmonoids under cul- 
ture is to be found in the abundant and constant introduction of atmospheric air 
into the living water; in other words, abundant and constant aeration. 

Can deeply infected fishes be cured and saved? I doubt it. I have never 
seen that surface wounds and abrasions of the skin healed; fungus invariably 
assailed the injured places and extended over the neighboring areas more and 
more until there ensued weakness, difficulty of moving, and lastly death, while 
deep wounds, bites, thrusts, and cuts were often found healed and leaving scars. 

Costia lives and increases on the skin and on the gills and destroys their tissue. 
Cure is always possible in the beginning of the infection, and the following phe- 
nomenon may be pointed out: All the fishes presenting a suspicious appearance — 
i. e., showing signs of weakness and discoloration and refusing food — were taken 
up by me with a gauze hand net and washed out in the water flowing from the 
hatching troughs. There was always water around the hatching boxes 3, 5, to 
10 centimeters deep, according to the height of water in the pond. Here all 
around the breeding troughs and in the narrow waterflow to the pond there came 
again and again small fishes, mostly S. jonlinalis of the same size as in the boxes, 
about 50 in June, and these seemed to be quite healthy, catching greedily at the 
crustaceans falling from the boxes. As it was impossible for them to come 
through out of the boxes, either these were cured fishes or I have taken unin- 
fected fishes out of the boxes. 

It need not be mentioned that CosHa spreads more rapidly when the fry are 
crowded and that the rise of temperature above 10° C. accelerates the progress 
of the infection and its communication. 



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